An experimental investigation of a deformable flat rough surface being in contact with a hard smooth ball indenter was presented to evaluate the effect of thickness on the flattening of surface asperities. A novel method to determine the asperities flattening was deployed. The method uses a matching and stitching technique for analyzing the 3D measurement surface data. Two different materials of flat surfaces, aluminium and brass, were used. Results were presented for very thin and thick specimens, showing that there is no effect of thickness on the deformation degree of the surface asperities. Comparisons were made with the published models for predicting the deformation degree of the contacting surface and the results obtained confirm very well the model.